Line by Line Walkthrough

quit = Qt::PushButton.new('Quit')

This time, the button says Quit and that's exactly what the program will do when the user clicks the button.

quit.resize(75, 30)

We've chosen another size for the button since the text is a bit shorter than "Hello world!". We could also have used Qt::FontMetrics to set right size, or let Qt::PushButton choose a reasonable default.

quit.setFont(Qt::Font.new('Times', 18, Qt::Font::Bold))

Here we choose a new font for the button, an 18-point bold font from the Times family. It is also possible to change the default font for the entire application, using Qt::Application::setFont().

Qt::Object.connect(quit, SIGNAL('clicked()'), app, SLOT('quit()'))

Qt::Object::connect() is perhaps the most central feature of Qt. Note that connect() in this context is a static function in Qt::Object. Do not confuse it with the connect() function in the Berkeley socket library.

This connect() call establishes a one-way connection between two Qt objects (objects that inherit Qt::Object, directly or indirectly). Every Qt object can have both signals (to send messages) and slots (to receive messages). All widgets are Qt objects, since they inherit Qt::Widget, which in turn inherits Qt::Object.

Here, the clicked() signal of quit is connected to the quit() slot of app, so that when the button is clicked, the application quits.